1. Technical Field
The present invention relates to machine tools furnished with a table atop which a workpiece is carried, a support structure for supporting the table allowing it to swivel about a horizontal swivel center axis, and a swivel drive mechanism for swiveling the table upon the swivel center axis to index the table to a specific swivel angle position.
2. Description of the Related Art
Such machine tools known to date include the example disclosed in Japanese Unexamined Patent App. Pub. 10-180523. This machine tool primarily is made up of: a machine column provided upright as appropriate on the installation surface; a horizontal slide placed on the upper face of the machine column movably right and left in the horizontal plane; a vertical slide placed on the upper face of the horizontal slide movably back and forth in the horizontal plane; a main spindle disposed so that its own axis is perpendicular, for retaining a tool; a spindle head provided on the front face of the vertical slide to freely move perpendicularly, for supporting the main spindle free to rotate axially; and a rotatable table supported on the front face of the machine column allowing it to swivel about a swivel center axis paralleling the back-and-forth orientation, and configured free to rotate upon the rotational center axis paralleling the perpendicular orientation.
This machine tool also has a horizontal slide guide mechanism for guiding the horizontal slide movement along the right-and left orientation, a vertical slide guide mechanism for guiding the vertical slide movement along the back-and-forth orientation, a spindle head guide mechanism for guiding the spindle head movement along the perpendicular orientation, a horizontal slide feed mechanism for moving the horizontal slide in the right-and-left orientation, a vertical slide feed mechanism for moving the vertical slide in the back-and-forth orientation, a spindle head feed mechanism for moving the spindle head in the perpendicular orientation, and a rotational drive mechanism for rotating the mains spindle upon its own axis.
The rotatable table comprises a rotating member on which a workpiece is carried, a disc-shaped axially rotatable support member disposed so that its own axis coincides with the swivel center axis, an axially rotatable rotary shaft disposed centered on the support member axis, with a first end of the rotary shaft affixed to the back face of the support member, and a table body for supporting the rotating member rotatably upon the rotational center axis.
The support member is supported by the machine column free to rotate axially owing to the engagement between the support member outer periphery and the machine column front face, and the rotary shaft is supported by the machine column via a bearing free to rotate axially, so that the axial rotations of the support member and the rotary shaft allow the rotating member and the table body to swivel upon the swivel center axis. Meantime, the rotating member, as well as the support member and the rotary shaft, are configurationally driven and rotated by a driving device.
The horizontal slide, vertical slide and spindle head in such a machine tool are moved by the feed mechanisms in the predetermined feed orientations, while guided by the guide mechanisms, and simultaneously the main spindle is rotated upon its own axis by the rotational drive mechanism, so that the workpiece carried on the rotatable table (the rotating member) is processed by the tool held in the main spindle.
During processing, the rotating member is rotated upon the rotational center axis by the driving device, and is indexed to a specific rotational angle position, and the table body is swiveled upon the swivel center axis by the deriving device, and is indexed to a specific swivel angle position.
In this conventional machine tool, however, the moment originating in the weight of the rotatable table itself and of the workpiece carried on the rotatable table causes the swivel center axis to downwards tilt out of the horizontal (that is, the rotating member, table body and rotary shaft are sloped), and thus prevents high precision processing of the workpiece, because with the rotational table protruding from the machine column, only one end of the rotational table is supported.
Specifically, along with the upsizing of the rotatable table, its weight increases, the distance between a portion, supported by the machine column, of the rotatable table and the center of gravity of the rotatable table lengthens, and a heavier and heavier workpiece is carried on the rotatable table, with the result that the greater moment is produced to further prevent the high precision processing.
In addition, although the life of the bearing for supporting the rotary shaft of the rotatable table structurally depends on the load and moment applied on the bearing, the moment has more significant effect on the bearing, compared with the load. For this reason, the larger the moment applied to the bearing, the shorter the bearing life.
The present invention is directed to solving these problems, and an object of the invention is to make available a machine tool that prevents the lower processing precision and shorter bearing life resulted from the table tilting that the moment produced by the weight of the table itself causes.